Project Summary/Abstract Simian virus 40 (SV40) encodes a powerful oncoprotein, large T antigen, that is capable of inducing tumors in animals and transforming cells in culture. T antigen acts in part by inhibiting the action of two tumor suppressor proteins, members of the retinoblastoma (Rb) family, and p53. Several lines of evidence indicate that T antigen has additional functions that contribute to transformation as well. Most studies of SV40 transformation have made use of cultured cells. In this application we propose to study the effects of T antigen expression on the growth-arrested enterocytes and continuously cycling progenitor cells of the mouse small intestine. Because the cycling cells residing in the crypts and the terminally differentiated cells occupying the villi can be isolated, this system allows a combined genetic and biochemical approach to dissecting T antigen's transforming functions. We have found that the transformation of terminally- differentiated enterocytes is dependent on the transcription factor E2F2. In contrast normal intestinal crypt cell proliferation does not require any of the activating E2Fs. The goals of this application are to: (1) determine if T antigen action on the Rb and p53 pathways sufficient for transformation of crypt epithelial cells; (2) assess the mechanisms allowing cell proliferation to proceed in normal crypts in the absence of E2F1-2-3 and, (3) determine whether the activating E2Fs (E2F1, E2F2, and E2F3a) required to establish repression of E2F-target genes?